Colour assessment - ECCA guidelines, final, 6.5.2013 · To ensure tonal consistency, all material...

Colour assessment of prepainted metal

ECCA guidelines

Why is colour management

important?

The directionality of

metallics should be

consistent especially when

panels are cut to size

To ensure tonal

consistency, all

material should come

from the same

production batch

Colour in practice – what does it

mean for the end user / client?

If different materials are used

next to each other (e.g.

powder coated and

prepainted), colour matching

of the powder coated

components should be done

to an actual sample of the

cladding material – not to a

theoretical value or a RAL

reference.

Colour: basic principles

Basic principles

Colour

• Colour stimulus consists of three components

• The light source sending out light waves (electromagnetic radiation) and the

surrounding conditions (environment around the light source and the object)

• The observed object that reflects a part of the light waves

• The observer that detects/the eyes that absorb the light waves reflected on the

surface of the object

Basic principles

• The light source

• The light source emits electromagnetic radiation at different wavelengths that all

relate to a different colour. All visible light is a mixture of wavelengths, each

corresponding to a different colour, which is specific to a light source. Therefore, a

colour of an object can seem to vary under different light sources.

• The object

• Any object reflects certain wavelengths of the incoming light and absorbs the

others. Therefore, an object that reflects green light waves and absorbs the others

looks green. Other aspects that affect the colour observation are shape and

surface. The object can be embossed or textured, or have metallic, pearlescent or

phosphorescent effects that all influence our experience of colour.

• The observer

• The defining observer of colour is the human eye. Thus, the colour experience is

highly subjective. The ability of the human eye and brain to detect and handle

colours is individual and depends on age, gender, inherited traits, and even mood.

Colour characterisation


What do we measure?

• Different colour spaces can be used to define a colour. Please note that the

colour observation is always dependent of illuminant and observer (e.g.

D65/10°)

The colour – CIELab

• The CIE L*a*b* colour space is the most used

in coil coating industry

• It defines the colour coordinates L*, a* and b*

� L* represents the colour lightness

� a* represents the colour green-red

dimension

� b* represents the colour blue-yellow

dimension


The colour – CIELCh

• CIELCh uses the same colour space than CIELab,

defining it with three colour coordinates L*, C*

and h*

� L* represents the colour lightness

� C* = represents Chroma or saturation of the

colour. Its value can be obtained from CIELab

by C* = �∗� + �∗�

� h* axis represents hue ranging from 0 (grey)

in the centre of the circle to 100 (saturated)

around the edge.

� All possible saturated colours are present

around the edge of the circle. The units are

expressed as degrees from 0° (red) through

90° (yellow), 180° (green), and 270° (blue)

back to 0°.


Colour measurement –

differences of geometry

Directional geometry

c) 45/0° d) 0°/45

Diffuse geometry

a) 8°/D (0°/D) b) D/8° (D/0°)

Multiangle


Colour measurement -

spectrophotometry

• Different types of equipment

available

• Colorimeter, spectrophotometer,

spectroradiometer

Colorimeter

Minolta: CR 400, D/0°°°°

Spectrophotometer

X-Rite: 964,

Spectrophotometer

X-Rite: VS450,45/0°

Spectrophotometer

BYK Gardner: BYK-mac, multiangle

Spectrophotometer

HunterLab: UltraScan, D/8°°°°

45/0°


Colour measurement – differences of measurement

• Depending on the equipment used

• Difference between the producers for the same kind of equipment

• More important with fabrication time

• Difference between one equipment family and another explained e.g.

by different parameters

• sphere diameter, light source (tungsten, xenon, diode, etc.), pulsed or continuous

light, etc.

• Depending on the geometry used

• L*a*b* absolute values depend on the geometry used

• Always small dispersion in absolute value due to equipment

calibration

• Mandatory to observe all the equipment conditions before

concluding a difference of colour


Metamerism

• In some cases two prepainted

samples can look exactly the

same under one light source,

but when transferred under a

different light source, their

visual appearance differs

significantly.

• This is called metamerism.

Appearance

• The final appearance of

prepainted metal depends

also on the type of resin

used, film thickness, gloss,

and the surface texture

(structured, granulated,

embossed, metallic,…)


smooth orange peel

grained structured

embossed wrinkled

Colour management

Colour management

Colour difference ∆E*

• The colour difference between a reference and a sample is

determined by the deviation on the different axes: ∆L*, ∆a*,

∆b*

• ∆E* (colour deviation) =

• Visual evaluation is necessary because ∆E* criteria can be

insufficient (metallized colours, colours with high whiteness,

limit of the equipment sensitivity, non-uniformity of CIELab

colour space…)

• Colour difference is always measured from 2 physical samples

(never with absolute values as references)

• It is practically impossible to have a ∆E* 0.0 colourmatch, a

small difference of ∆E* is normal and expected.

∆�∗� + ∆�∗² + ∆�∗²


The colour - CMC

• CMC is a modification of ΔE of CIELab developed

by the Color Measurement Committee of the

Society of Dyers and Colorists. It is defined in

AATCC Test Method 173, “CMC: Calculation of

Small Color Differences for Acceptability.”

• The CMC calculation mathematically defines an ellipsoid around a defined and

agreed standard colour. The dimensions of the ellipsoid depends on the colour shade

and thus can be expected to better correspond to the sensibility of the human eye.

• This ellipsoid consists of a semi-axis that corresponds to the attributes of hue (h),

chroma (C), and lightness (L)

• It represents the area of acceptance in relation to the standard, the same way the

CIELab "box" defines acceptable difference limits

• The size of the ellipsoid varies depending on its position in the colour space and its

shape is influenced by the CMC ratio l/c lightness/chroma. The c (chroma) tolerance

is usually smaller, since human eye is able to detect smaller shifts in chroma than in

lightness. Also, in the orange region, ellipsoids are narrower, while in the green

region, ellipsoids are wider.

Colour category definition

• Colour classification by category is

obtained from the colour

parameters L*, a*, b*, C*

- Category 1: light colours

- Category 2: medium colours

- Category 3: dark colours

- Category 4: sharp/saturated colours

- Category 5: metallized colours

Colour management

NOTE! Colours that do not fall into any category are classified by

the paint supplier.

C* ≤ 10 Category 1

10 < C* ≤ 20 Category 2

20 < C* ≤ 30 Category 3

C* > 30 Category 4


C* ≤ 25

and -11 < a* < 11

and -5 < b* < 25

C* ≤ 30

and -16 < a* < 16

and -5 < b* < 25

C* > 30 Category 4


C* > 30 Category 4

L* > 80

60 < L* ≤ 80

L* ≤ 60

Category 2

Category 3

Examples of colour categories

Colour management

Colour difference tolerances- recommendations

Colour tolerance

recommendations

General tolerances to be applied using 45/0°, 0°/45 and D/8°

geometries with spectro equipment

Note! The supplier has the final responsibility of defining the colour category of his

products. This applies particularly for special, demanding colours and cases where the

colour is close to a limit between two categories, or where a colour shade falls out of

any category

CIELab

DECMC

CF = 1,00

L:C= 2,00

Category 1: light colours ΔE* ≤ 1,0

Category 2: medium colours ΔE* ≤ 1,3

Category 3: dark colours ΔE* ≤ 1,5*

Category 4: sharp/saturated colours ΔE* ≤ 1,5*

Category 5: metallized colours

* ΔE* ≤ 2,0 can be accepted for specific colours

visual control required

Colour tolerance recommendations

Geometry used

45/0° or 0°/45

D/8°

ΔE* ≤ 1,0

ΔECMC ≤ 1,0

ΔE* ≤ 1,5*

• Metallized colours: Interaction light-material

Colour tolerance

recommendations

• ΔE* measurement can be disturbed by

interaction between the incoming light

and the surface of the metallic flake.

Visual control remains important.

• Specular reflection on metallic fillers:

mirror effect, variable effect on the

reading depending on the orientation

of the angle

Guidelines to ensure colour

consistency

• Different production batches

should not be used next to each

other even if they are of the same

colour AND even though all coils

from different orders have the

same tolerance compared to the

colour master standard.

Colour tolerance

recommendations

• ΔE* guaranteed depends

on the colour category

Examples of colour differences

ECCA Green

∆L=-5 ∆L=+5

∆a=+5∆b=+5

∆a=-5 ∆b=-5

ECCA 50% Grey

∆L=-5 ∆L=+5

∆a=+5∆b=+5

∆a=-5 ∆b=-5

RAL 7035

Light grey

∆L=-5 ∆L=+5

∆a=+5∆b=+5

∆a=-5 ∆b=-5

RAL 1015

Light ivory

∆L=-5 ∆L=+5

∆a=+5∆b=+5

∆a=-5 ∆b=-5

RAL 7000

Squirrel Grey

∆L=-5 ∆L=+5

∆a=+5∆b=+5

∆a=-1 ∆b=-1

∆L=-1 ∆L=+1

∆b=+1 ∆a=+1

∆a=-5 ∆b=-5

Colour matching and specification

Colour match and specification

Procedure

• The colour master is the colour matched by the paint supplier

• No colour match is based on theoretical values

• A physical colour match is the closest proposition to the colour

master: the chemical paint composition is perfectly fixed and

reproducible

• One colour match is kept by the customer and one by the coil coater

• When accepted by the customer, the physical colour match is

used as reference for mass production by the coil coating line

and in all discussions or possible claims with the customer

• A physical colour master is mandatory. A colour master on

paper/resin/other substrate/other paint system is never

accepted as reference for mass production!

Customer specifications are to be written in common

agreement between supplier and the customer

• Colour measurement equipment shall be clearly defined

• Colour evaluation according to EN 13523-3, visual evaluation

according to EN 13523-22

• Tolerance on the colour (∆E*) is defined by the supplier, but

can be discussed with the customer

• Tolerance on each axis (L*, a*, b*) can be fixed for demanding

colours or applications (e.g. domestic appliances, electronic

devices etc.)


Mechanical

performance level

(flexibility, adhesion,…)

Conditions of

installation

(formability,…)

Performance after installation

(UV & corrosion resistance,

abrasion,…)

Desired appearance

(Colour, gloss, texture…)

Type of resin (epoxy,

PUR, PVDF,…)

Substrate type


PUR, PVDF,…)

Film building

capacity


PUR, PVDF,…)

Film thickness

Appearance (structured,

granulated,…)

Film thickness

Colour, Gloss

Appearance (structured,

granulated,…)

Customer specification Colour master

To develop a specification for a prepainted product, the

following steps are recommended:

Colour matching procedure should be the last step in the

definition of specifications for a prepainted product.


Colour assessment - ECCA guidelines, final, 6.5.2013 · To ensure tonal consistency, all material...

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